The comprehensive review on allelopathy (Rice, 1979, 1984) has been largely responsible for the evolution of allelopathy as an independent branch of chemical/physiological ecology. The allelopathic research during the last four decades drew attention to different facets of the interactions among the constituents of habitat, calling for an understanding of the role of allelopathy under different habitat conditions. In view of this, we have reviewed the existing information on allelopathic interactions in aquatic habitats with special reference to algal allelopathy. This review has been mainly confined, therefore, to different aspects of algal allelopathy such as allelopathic interactions in algae, algal toxins, bioassays, and implications of algal allelopathy. In spite of the large number of reviews on allelopathy (see section III), no independent review appears on algal allelopathy. Although there were reports of toxins from cyanobacteria and other algae, no appreciable attempt was made to implicate algal toxins in allelopathy under field conditions. Knowledge of chemistry and biology of allelochemical can help in their potential use in controlling plant diseases and weeds. Therefore, it is urgent to study algal toxins for their involvement in ecological phenomena such as succession, for their uses as herbicides, weedicides, and pesticides, for their uses in solving some of the problems of algal ecology, and for their involvement in applied aspects. Algal allelopathy is a manifold ecological/physiological phenomenon. Chemicals contributed by the alga can affect (1) other algae in its vicinity, (2) its own growth (i.e., autotoxicity), (3) microbes associated with it, (4) higher plants in its vicinity, and (5) accumulation and availability of nutrient ions which can influence the distribution, growth and establishment of other algae, microorganisms, and plants. However, to establish algal allelopathy of ecological relevance, it is essential to demonstrate the involvement of allelopathy under field conditions. Further, comments should be made on residence time, biological active concentration, mode of renewability, static and dynamic availability of allelochemical, and its variation, if any, with season, site, habitat, and environmental factors. Further, many algae, especially blue-green, influence the zooplankton population. Is it just a toxic effect of blue-green algae (BGA), or can it be included under allelopathy? This point has been debated and is discussed in the present article.
